1. Field of the Invention
The present invention relates to a wireless communication system and a wireless communication apparatus. In particular, the invention relates to a method of exchanging a cryptography key between wireless communication apparatuses.
2. Description of Related Art
Wireless data communications are more adapted to mobile communication environments than wired data communications. On the other hand, the wireless data communications are disadvantageous in that communication data can be easily stolen or tampered. As one popular solution to this problem, in a wireless communication system transmitting/receiving communication data through a wireless interface, the communication data is encrypted and then transmitted/received.
According to this method, even if stolen midway through a transmission path, the encrypted communication data cannot be decrypted unless a cryptography key used for encryption is obtained. Thus, a high security level of the wireless communication system is ensured by encrypting communication data.
Security specification of a wireless USB (Universal Serial Bus) is described in “Wireless Universal Serial Bus Specification Revision 1.0”, Chapter 6, pp. 119-136. The wireless USB employs a common key encryption system, and allows transmission/reception encrypted communication data between a WUSB host and a WUSB device with a common private key. The communication data is encrypted based on AES-128CCM (Advanced Encryption Standard-128 bit Counter-mode with CBC MAC Protocol).
To elaborate, information called “Connection Context” is shared between the WUSB host and the WUSB device. The connection context includes three kinds of information: a host ID (CHID: Connection Host ID), a device ID (CDID: Connection Device ID), and a connection key (CK: Connection Key). The connection context varies from one pair of WUSB host and WUSB device to another.
A PTK (Pair-wise Temporal Key) is generated based on a connection key CK shared between the WUSB host and the WUSB device. The communication data is encrypted and decrypted between the WUSB host and the WUSB device based on the pair-wise temporal key PTK.
As described above, the wireless USB requires sharing of connection key CK between the WUSB host and the WUSB device. Various methods have been proposed for sharing the connection key CK. To be specific, there have been proposed a method of transferring a cryptography key from a host to a device via a connection cable as disclosed in Japanese Unexamined Patent Application Publication No. 2005-318079, and a method of transferring a cryptography key with a communication mode such as an infrared or optical communication mode where light travels in a straight line, or with weak radio waves as disclosed in Japanese Unexamined Patent Application Publication No. 2003-283481.
The method with use of the connection cable as disclosed in Japanese Unexamined Patent Application Publication No. 2005-318079 needs to reduce a distance between the WUSB host and the WUSB device to a distance shorter than the connection cable length. Therefore, if it is difficult to bring the WUSB host and the WUSB device closer together due to any obstacle therebetween or if the WUSB host or WUSB device is not easy to carry, a problem arises.
On the other hand, the method of transferring a cryptography key with a communication mode such as an infrared or optical communication mode where light travels in a straight line, or with weak radio waves as disclosed in Japanese Unexamined Patent Application Publication No. 2003-283481 has a problem that the cryptography key is transmitted by radio waves propagating in the air, so a fear about decryption of the cryptography key, spoofing, or tampering of the cryptography key through radio intercept cannot be completely cast aside.
Incidentally, these problems are not involved in the above WUSB wireless communication system alone. That is, the problems would occur in all wireless communication systems that need to exchange a so-called cryptography key used for encrypting/decrypting communication data between two wireless communication apparatus to share the cryptography key.